The German zookeeper who hand-reared Knut the polar bear and helped turn the animal into a global celebrity has been found dead in his Berlin flat, police said today.

Thomas Dörflein was said to have been seriously ill for some time, and officers do not believe any foul play was involved.

Dörflein became Knut's surrogate parent when the cub was rejected by his mother, Tosca, after being born at the Zoologischer Garten Berlin in December 2006.

Thanks to round-the-clock care from the 44-year-old, who hand-fed Knut and slept in the same cage, the polar bear cub became the first at the zoo to survive past infancy for more than 30 years.

Knut-mania gripped the world last March when Dörflein accompanied the cub on his first public appearance in front of the global media.

The ensuing attention turned the bear and his keeper into media stars. Knut appeared on the cover of a German edition of Vanity Fair, and his popularity drew record numbers of visitors to the zoo.

Dörflein and Knut built a special rapport, and the bearded zookeeper was rarely seen without the white bear following close behind.

As Knut grew, however, it became too dangerous for Dörflein to play with him. Last November, the zoo decided it would be best if all contact between the two was cut off.

Not everyone had appreciated Dörflein's work. Some animal rights activists argued that Knut should have been killed rather than turned into a "domestic pet", and others claim the animal is showing signs of disturbed behaviour.

Last year, Dörflein, a father of three, was awarded the Medal of Merit by the city of Berlin for his work in rearing Knut and raising the zoo's profile.